1. Field of the Invention
The present invention relates to methods of positioning electrodes in contaminated porous medium for applying the electricity through the medium to induce the electrokinetic phenomena in the medium and maintaining the medium pH in conditions of an applied electric field to enhance extraction of contaminants from the medium by the plants grown in the medium.
2. Background of the Related Art
Phytoremediation is a relatively new technology for cleanup of contaminated soil that uses green plants to extract, contain, or render harmless an environmental contaminant. Research is discovering that more and more plants are capable of accumulating extremely high concentrations of contaminants in their roots or aboveground shoots. Some of these plants are metal hyperaccumulators which can contain a metal concentration in their shoots that is up to 1000 times greater than normal species. Three subsets of the phytoremediation technology are already under development: (i) phytoextraction--the use of metal-accumulating plants, which can remove metals from soil into the harvestable parts of the plant; (ii) rhizofiltration--the use of plant roots to remove contaminants from polluted aqueous streams, and (iii) phytostabilization--the use of plants to stabilize contaminants such as toxic metals in soils and to prevent their entry into ground water.
A number of laboratory and field scale studies have demonstrated the potential of several hyperaccumulators for extraction or destruction of contaminants. Brassica juncea (mustard) has been shown to be useful for lead extraction from soil (up to 16,000 ppm lead accumulated in the plant and also the roots concentrate metals 131 to 563-fold, on a dry weight basis, above initial solution concentrations). Helianthus annuus L. (sunflower) accumulates copper, cadmium, chromium, nickel and zinc from aqueous solutions. The aquatic plant Limnathemum cristatum accumulates chromium and cadmium from solutions at metal concentrations up to 2.0 mg/l. Prairie grasses enhance bioremediation of polyaromatic hydrocarbons by improving aeration in soil and degradation capability in the rhizosphere. Alfalfa enhances microbial destruction of hydrocarbons in the rhizosphere. Populus spp. (poplar) trees are capable of enhancing evapotranspiration and mineralization to manage water and priority pollutants. Further, canola, wild brown mustard, and tall fescue are beneficial for reducing water-extractable boron and total selenium in soil.
The use of genetically engineered plants have demonstrated that an increased metal tolerance could be achieved in plants. For instance, high mercury tolerance was achieved by the introduction of metallothioneins and a semi-synthetic gene encoding MerA (bacterial mercuric ion reductase). A mutant of Pisum sativum was able to accumulate 10-100 fold more iron than the wild type.
Phytoremediation is an attractive approach for cleanup of soil because it is inexpensive and requires little maintenance. However, its application is limited to surface contamination only, because the cleanup depth is strictly determined by the length of the plant roots. It is a passive technology in terms of contaminant transport, the movement of contaminants in the soil is induced exclusively by a slow plant root suction and thus the efficiency of removal of contaminants depends on the extension of the plant roots in the soil subsurface.
Electroosmosis is defined as the mass flux of a fluid containing ions through a stationary porous medium caused by the application of an electrical potential. The fluid moves through the voids in the porous medium (e.g. soil) called pores. Each pore has a thin layer of charged fluid next to the pore wall having a typical thickness of between about 1 and about 10 nanometers. The thin layer of charged fluid next to the pore wall is present to neutralize the charge (typically a slight negative charge) on the surface of the soil particle that forms the pore wall. Fluid movement occurs in soil pores because of the charge interaction between the bulk of the liquid in the pore and the thin layer of charged fluid next to the pore wall. Under the influence of a DC electric field, the thin layer of charged fluid moves in a direction parallel to the electric field. Large amounts of liquid may be transported along with the thin layer of charged fluid as well as contaminants or other species contained within the liquid. Electroosmosis is most effective in fine-grained soils, such as silty or clayey soil, where hydraulic transport of water, such as pumping or irrigating, is not feasible. Electroosmosis is used in electrokinetic remediation to remove non-charged organic contaminants by electrical pumping of contaminated pore water from soil.
Electromigration is defined as the mass flux of a charged ionic or polar species within a liquid or solution from one electrode to another electrode. Electromigration and electroosmosis may occur simultaneously and are the dominant mechanisms through which conventional electrokinetic transport processes occur.
Electroosmosis has been used as a method for dewatering soils and sludges. The fundamentals of this method were established tbrough the work of Casegrande and Grey. During the 1970's, electrokinetic metal recovery was used as a method for mining metals such as copper. These processes involved inserting electrodes enclosed within porous enclosures or wells into the ground. The enclosures are then filled with an electrolyte, typically an acid.
One recent application in which electrokinetic transport of materials has found practical use is the electrokinetic remediation of contaminants in soil. Electrokinetic remediation, frequently referred to as either electrokinetic soil processing, electromigration, electrochemical decontamination or electroreclamation, uses electrical currents applied across electrode pairs placed in the ground to extract radionuclides, heavy metals, certain organic compounds, or mixed inorganic species and organic wastes from soils and slurries. Electrokinetic remediation of contaminated soils can be applied to fine grain soils and operates efficiently in soils with hydraulic conductivity lower than 10.sup.-4 cm s.sup.-1. In such low hydraulic conductivity conditions, the hydraulic transport of fluids, e.g., pumping or irrigating, is not feasible.
During electrokinetic processing, water in the immediate vicinity of the electrodes is electrolyzed to produce H.sup.+ ions at the anode and OH.sup.- ions at the cathode, causing the pH of the soil to change, according to the following equations.
Anode Reaction EQU 2H.sub.2 O.fwdarw.O.sub.2 +4e.sup.- +4H.sup.+ Equation (1)
Cathode Reaction EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- Equation (2)
If the ions produced are not removed or neutralized, these reactions lower the pH at the anode and raise the pH at the cathode. Protons formed at the anode migrate towards the cathode and can aid contaminant removal by increasing metal extraction. In contrast, the hydroxyl ions formed at the cathode do not migrate as efficiently as protons and can increase the soil pH in the cathode region, as high as a pH of 12, and cause deposition of tinsoluble species, thereby forming regions of high electrical resistivity.
U.S. Pat. No. 5,785,735 (Raskin) discloses a method for manipulating a soil environment by applying an electric field in the soil to increase the mobility and availability of metals to the root systems of plants capable of absorbing and retaining the metals. Raskin also discloses that the adjustment of soil pH below pH 5.0 by addition of organic or inorganic acids can be utilized to enhance the metal availability to the plants. However, there is a central dilemma regarding the survivability and growth of plants in conditions of low pH or where an electric field is applied through the soil.
There remains a need for a phytoremediation process that enhances the accumulation of metals in plants. More particularly, there is a need for a phytoremediation process that removes metal from soil below the root zone of plants and maintains healthy plant growth.